Flagging GMail Emails for Thunderbird to Download

I lost about 6 months of emails while using Netscape email. I switched to Thunderbird this morning. I managed to save all of the Netscape emails and folders. However, I am still missing the emails that I lost while using Netscape. The missing emails are still online in GMail. Thunderbird will not download those emails because it "knows" they're old emails. I tried marking the missing emails in GMail as Unread, but Thunderbird still knows they're old emails, and will not download them. I tried using the "recent:" prefix on my email user name, but that feature only receives the last 30 days of emails. Is there a way to mark the missing emails in GMail to trick Tunderbird into thinking they're new so they will be downloaded. I am using Thunderbird as my POP3 email client.